Nickel hydroxide composition with pectin binder

ABSTRACT

An active electrode composition comprising a nickel hydroxide material and a pectin. Preferably, the pectin comprises a citrus pectin.

RELATED APPLICATION INFORMATION

This application is a continuation of U.S. patent application Ser. No.09/501,944 filed on Feb. 10, 2000. U.S. patent application Ser. No.09/501,944 is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to alkaline electrochemical cells. Inparticular, the present invention relates to active compositions forelectrodes of alkaline electrochemical cells.

BACKGROUND OF THE INVENTION

Rechargeable electrochemical cells may be classified as “nonaqueous”cells or “aqueous” cells. An example of a nonaqueous electrochemicalcell is a lithium-ion cell which uses intercalation compounds for bothanode and cathode, and a liquid organic based or polymer electrolyte.Aqueous electrochemical cells may be classified as either “acidic” or“alkaline”. An example of an acidic electrochemical cell is a lead-acidcell which uses sulfuric acid as the electrolyte, lead dioxide as theactive material of the positive electrode, and metallic lead, in ahigh-surface area porous structure, as the negative active material.

Examples of alkaline electrochemical cells are “nickel-based” alkalinecells. These cells use an alkaline electrolyte (such a potassiumhydroxide) and nickel hydroxide as the active material for the positiveelectrode. Nickel hydroxide has been used for years as an activematerial for the positive electrode of alkaline batteries. The reactionsthat take place at the positive electrode of a nickel-based rechargeableelectrochemical cell are reversible and include the following chemicalreaction:

At the positive electrode, Ni(OH)₂ is oxidized to NiOOH during thecharge operation. During discharge, the NiOOH is reduced to Ni(OH)₂.Examples of such nickel-based alkaline batteries include nickel-metalhydride cells (Ni-MH), nickel cadmium cells (Ni—Cd), and nickel-zinccells (Ni—Zn). Ni-MH cells used negative electrode using a hydrogenstorage alloy as the active material. The hydrogen storage alloy iscapable of reversible electrochemical storage of hydrogen.

In general, Ni-MH cells utilize a negative electrode that is capable ofreversible electrochemical storage of hydrogen, and a positive electrodeof nickel hydroxide material. The negative and positive electrodes arespaced apart in the alkaline electrolyte.

Upon application of an electrical potential across a Ni-MH cell, thehydrogen storage alloy of the negative electrode is charged by theelectrochemical discharge of hydrogen and the electrochemical generationof hydroxyl ions:

The negative electrode reactions are reversible. Upon discharge, thestored hydrogen is released to form a water molecule and release anelectron. (In a Ni—Cd cell, cadmium metal is the active material in thenegative electrode).

The active electrode material for both the positive and negativeelectrodes is usually affixed to a conductive substrate to form thepositive and negative battery electrodes. One way to affix the activematerial to the conductive substrates is to first make the activematerials into a paste by adding a small amount of binder and thenapplying this paste to the substrate. The present invention is directedto a new active material composition including a binder comprising amonosaccharide, a disaccharide, a pectin or a molasses. Electrodesincorporating this active composition have improved electrochemical andmechanical properties.

SUMMARY OF THE INVENTION

An aspect of the invention is an active composition for an electrode ofan alkaline electrochemical cell, comprising: an active electrodematerial; and a binder comprising a monosaccharide, a disaccharide, apectin or a molasses.

Another aspect of the invention is an electrode for an alkalineelectrochemical cell, comprising: an active composition including: anactive electrode material; and a binder comprising a monosaccharide, adisaccharide, a pectin or a molasses.

Another aspect of the invention is an alkaline electrochemical cell,comprising: at least one positive electrode; at least one negativeelectrode; and an alkaline electrolyte, the positive electrode and/orthe negative electrode having an active material composition including abinder comprising a monosaccharide, a disaccharide, a pectin or amolasses.

Preferably, the binder comprises a pectin.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein is an active material composition for use in anelectrode of an alkaline electrochemical cell. Generally, the activematerial composition comprises an active electrode material and abinder. The binder comprises a monosaccharide, a disaccharide, a pectinor a molasses. In addition, the binder may be a mixture of one or moreof a monosaccharide, a disaccharide, a pectin or a binder.

Examples of a monosaccharide include glucose, fructose, and galactose.An example of a disaccharide is maltose. An example of a pectin is afruit pectin such as a citrus fruit pectin and an apple pectin. Anexample of a molasses is the liquid left after sucrose has been removedfrom the mother liquid in sugar manufacture.

Generally, the active electrode material may either be a positiveelectrode material or a negative electrode material. Examples ofpositive electrode materials include nickel hydroxide, manganesedioxide, zinc oxide, nickel oxide, manganese hydroxide, copper oxide,molybdenum oxide, carbon fluoride, etc. Examples of negative electrodematerials include zinc, cadmium hyroxide, hydrogen storage alloys, etc.

Preferably, the active electrode material of the present invention is anickel hydroxide material. It is within the spirit and scope of thisinvention that any nickel hydroxide material be used as the activematerial. Example of nickel hydroxide materials are provided in U.S.Pat. Nos. 5,348,822 and 5,637,423, the contents of which areincorporated by reference herein.

As described, the active composition comprises an active electrodematerial and a binder comprising a monosaccharide, a disaccharide, apectin or a molasses. Preferably, the binder comprises a pectin or amolasses. In one embodiment of the invention, the binder comprises apectin. Preferably, the binder consists essentially of a pectin. Inanother embodiment of the invention, the binder comprises a molasses.Preferably, the binder consists essentially of a molasses.

The binder is preferably intermixed with the active electrode material.More preferably, the binder is intermixed with the active electrodematerial so as to form a paste. Hence, if the binder is in solid form,it is preferably dissolved in an appropriate solvent before being mixedwith the active electrode material. After, the binder is dissolved, theactive electrode material is added in order to form an active pastecomposition.

Preferably, the binder is between about 0.1% and 2.0% by weight of theactive material composition. More preferably, the binder is betweenabout 0.2% and about 0.8% by weight of the active material composition.Most preferably, the binder is between about 0.3% and about 0.5% byweight of the active material composition.

An example of an active electrode composition of the present inventioncomprises about 88.6 wt % nickel hydroxide, about 5 wt % cobalt, about 6wt % cobalt oxide, and about 0.4 wt % pectin.

Also disclosed herein is an electrode for use in an alkalineelectrochemical cell. The electrode comprises the active materialcomposition disclosed above. The active material composition is affixedto a conductive substrate in order to form the electrode. The conductivesubstrate is used as a support and as a current collector for the activematerial composition. The electrode is formed by first forming theactive composition. Mixing the active material with the binder (insolution) forms a paste which may be applied to the conductive substrateto form an electrode. After the paste is applied to the conductivesubstrate, the resulting electrode may then be allowed to dry. After theelectrode is dried, it may then be compacted so as to obtain therequired thickness. Compaction may be accomplished through the use ofone or more rolling mills.

The conductive substrate may be any electrically conductive supportknown in the art capable of holding the active material composition. Itis within the spirit and intent of this invention that any and all kindsof electrically conductive substrates may be used. Examples of substrateinclude foam, perforated metal, expanded metal, screen, matte, and ametal foil. Generally, any metal may be used as long as it is immunefrom corrosion at the pH and potential of the electrode. Examples ofmetals include nickel, nickel alloy, nickel plated steel, and nickelplated copper. Substrate for the negative electrode may further includecopper or copper alloy. In one embodiment, the substrate is in the formof a metal foam comprising nickel, nickel alloy, nickel-plated steel(i.e., steel which is plated with nickel), or nickel-plated copper(i.e., copper which is plated with nickel). Preferably, the metal foamcomprises nickel or a nickel alloy. Other forms of substrate (forexample, a nickel foil) may be used to make a thinner electrode. Oneembodiment of the electrode of the present invention is a nickelhydroxide electrode using an active material composition comprisingnickel hydroxide and pectin.

Also disclosed herein is an alkaline electrochemical cell comprising atleast one positive electrode, at least one negative electrode, and analkaline electrolyte. The positive electrodes and/or the negativeelectrodes may be formed using the active material composition of thepresent invention. That is, the positive electrodes may be formed usingan active positive electrode material and a binder comprising amonosaccharide, a disaccharide, a pectin or a molasses. Also, thenegative electrode may be formed using an active negative electrodematerial and a binder comprising a monosaccharide, a disaccharide, apectin or a molasses.

For example, a Ni-MH electrochemical cell may be formed by usingpositive electrodes having an active composition comprising a nickelhydroxide material and pectin. The negative electrodes comprise ahydrogen storage alloy as the active electrode material. Optionally, thenegative electrodes may, in addition, have an active composition formedusing a hydrogen storage alloy mixed with a pectin binder. Typically,the alkaline electrolyte is an aqueous solution of an alkali metalhydroxide such a potassium hydroxide, lithium hydroxide, sodiumhydroxide or mixtures thereof.

The addition of a pectin binder to an active electrode material, such asa nickel hydroxide material, improves the durability and cycle life ofthe electrode. Specifically, during the charging process of a sealednickel-metal hydride battery, the positive electrode reaches full chargebefore the negative and begins to evolve oxygen,2OH^(−----H) ₂0+½O₂+2e ⁻  (3)The evolved oxygen can oxidize the positive electrode and cause itsmechanical disintegration, thereby reducing the electrode's cycle life.In particular, the oxidation can reduce the adhesion and electricalconductivity between the active nickel hydroxide particles and thesubstrate, thereby increasing the electrode's resistance and reducingthe amount of power available for output.

The pectin binder is resistant to oxidation. Hence, while not wishing tobe bound by theory, it is believed that the pectin binder protects theelectrode from oxidation and deterioration, thereby increasing theintegrity and cycle life of the electrode. Furthermore, because it isresistance to oxidation, it is also believed that the pectin binderimproves both the particle-to-particle and particle-to-substrateadhesion. The improved adhesion provides for better electrical contact(i.e., improved electrical conductivity) between the active particlesand the substrate so as to reduce the internal resistance of theelectrode. Hence, less power is wasted due to internal dissipation andmore power is available to apply to an external load.

In addition to increasing power, the improved adhesion keeps the activeelectrode material together thereby reducing the chance that activematerial particulate may become loose and penetrate one of theseparators to cause an electrical short. The improved adhesion alsoincreases the flexibility of the electrode. Hence, the electrodes may betwisted, bent and wound more easily.

The improved adhesion also allows for making thinner electrodes. Thismay be accomplished by using a thinner substrate, such as a foilsubstrate, for supporting the active material. Because of the improvedadhesion, the active composition remains attached to the thinnersubstrate. Thinner electrodes may also be made by increasing the amountof force used when compacting the active material onto the substrate.Using thinner electrodes allows for a greater number of electrodes to beplaced within the case of the electrochemical cell. This also increasesthe power of the electrode and cell.

EXAMPLE

A first active positive electrode paste (the “control” material) wasprepared using 88.6 wt % Ni(OH)₂, 5.0 wt %, Co, 6 wt % CoO and 0.4 wt %PVA binder (polyvinyl alcohol) binder. The paste was affixed to a nickelfoam substrate to form a “control” positive electrode.

A second active positive electrode paste was prepared using 88.6 wt %Ni(OH)₂, 5.0 wt %, Co, 6 wt % CoO and 0.4 wt % pectin. The pectin is acitrus pectin. The paste was affixed to nickel foam substrates to form“pectin” positive electrodes.

A third active positive electrode paste was prepared using 88.6 wt %Ni(OH)₂, 5.0 wt %, Co, 6 wt % CoO and 0.4 wt % molasses. The paste wasaffixed to nickel foam substrates to form “molasses” positiveelectrodes.

Using the above identified “control”, “pectin” and “molasses” positiveelectrodes, three nickel metal hydride electrochemical cells were madeusing negative electrode comprising the same hydrogen storage alloy. Allof the cells were cycled (charged/discharged) using the same procedureand the peak power was measured at 50% DOD (“depth of discharge”) and at80% DOD. The results are presented in the Table. The peak power isprovided in Watts per kilogram (W/kg). TABLE Peak Power (W/kg) 50% DOD80% DOD Control 254 226 Pectin 274 238 Molasses 286 238

It is to be understood that the disclosure set forth herein is presentedin the form of detailed embodiments described for the purpose of makinga full and complete disclosure of the present invention, and that suchdetails are not to be interpreted as limiting the true scope of thisinvention as set forth and defined in the appended claims.

1. An active composition for an electrode of an alkaline electrochemical cell, comprising: a nickel hydroxide material; and a pectin.
 2. The active composition of claim 1, wherein said pectin comprises a citrus pectin.
 3. The active composition of claim 1, wherein said pectin is between about 0.1 and 2.0 weight percent of said active material composition.
 4. A positive electrode for an alkaline electrochemical cell, comprising: a nickel hydroxide material; and a pectin.
 5. The electrode of claim 4, wherein said pectin is a citrus pectin.
 6. The electrode of claim 4, wherein said pectin is between about 0.1 and 2.0 weight percent of said active material composition.
 7. An alkaline electrochemical cell, comprising: at least one positive electrode; at least one negative electrode; and an alkaline electrolyte, said positive electrode having an active material composition including a nickel hydroxide material and a pectin.
 8. The electrochemical cell of claim 7, wherein said pectin comprises a citrus pectin.
 9. The electrochemical cell of claim 7, wherein said pectin is between about 0.1 and 2.0 weight percent of said active material composition.
 10. The electrochemical cell of claim 7, wherein said negative electrode comprises a hydrogen storage alloy. 